1. Field of the Invention
The present invention relates to a portable, hand held probe in a Doppler fetal heart beat detection and monitoring system for detecting the fetal heart beat using Doppler ultrasound techniques and more specifically to a method and a noise controller within the probe capable for reducing break noise associated with the application of acoustic coupling gel to the probe and the momentary interruption in the probe/gel/skin interface associated with the movement of the probe across the skin during usage of the probe.
2. Description of the Related Art
The Doppler effect was first described in the 19th century by Christian Doppler, an Austrian scientist from Salzburg. A hand held ultrasonic Doppler fetal heart beat detection and monitoring system includes a probe for detecting the fetal heart beat and for providing an analog signal to a headset and/or to an auxiliary unit (hereinafter referred to as a Calc. unit). The probe includes one or more crystals that transmits and receives ultrasonic sound waves. In use, the detector is held against the mothers abdomen and directed towards the fetus. The transmitter crystal generates an ultrasonic wave that passes into the mothers body. The transmitted ultrasonic wave is reflected by the movement of the fetal heart as a reflected ultrasonic wave to the receiving crystal. The frequency of reflected ultrasonic wave is changed as a function of the velocity of movement of the fetal heart. This frequency shift is detected and processed by the probe into an analog signal that can be heard as the fetal heart beat through the headset and the speaker in the Calc. unit. The Calc. unit also processes the analog signal to derive a fetal heart rate and displays the same.
A probe 11 having a single energy level transmitter and a detector 51, volume controller 52 and power supply 54 of FIG. 2 is available from MedaSonics, Inc. as Part No. 101-0135-010. A Calc. unit suitable for use with probe 11 is available from MedaSonics, Inc., 47233 Fremont Boulevard, Fremont, Calif., 94538, and is identified as FETAL CALC. SPEAKER/HEART DISPLAY, Part No. 101-0238-010. A headset 10 compatible for use with probe 11 can also be purchased from MedaSonics, Inc., and is identified as HEADSET, Part No. 101-0008-010.
During the later stage of the first trimester and the early stage of the second trimester, the heart of the fetus is so small that the conventional hand held Doppler fetal heart beat detection and monitoring systems encounter difficulty in detecting the fetal heart beat due to the very low level of ultrasonic energy in the reflected ultrasonic wave from the fetal heart.
One approach to this problem was to provide a set of interchangeable probes where each probe emits ultrasound energy at a different ultrasonic frequency to improve the sensitivity of the probe in detecting the fetal heart beat. The disadvantage of this solution is that the user has to have easy and immediate access to the set of probes, the necessity of physically having to change the probes during the examination, the potential of one or more probes being damaged and the increase cost of having more than one probe.
Other approaches have been directed to methods of processing of the reflected ultrasonic wave by the detector in the probe to distinguish the low level fetal heart beat component from the noise component of the reflected ultrasonic waves.
Typically a medium, such as an aqueous based acoustic gel or petroleum based gelatin, is applied to the probe. The medium acts as an acoustic impedance matching interface between the probe and the insonated area, which is the skin surface. Application of the medium to the probe generates unwanted noise, referred to as break noise, which appears as a high amplitude signal component in the output signal of the detector and is heard as a loud sound in the head set or from a speaker in the Calc. unit. Break noise is also generated when the probe is moved across the skin surface causing the probe/medium/skin interface to be broken.